“Downhole equipment” is understood to refer to any tool, equipment or instrument that is used in a wellbore.
Data needs to be transmitted between downhole equipment and the surface for various purposes, for example:                monitoring performance of motors/pumps        transmission of control signals for control of valves        measuring device orientation and position        making physical measurements        
For motorised downhole equipment, data needs to be sent from below the equipment in a circuit that includes motor windings and the equipment's power cable. The rationale is that since there are already power cables present the cost of the solution using these should be proportionately less than one where you must supply the appropriate length of communications cable.
Therefore, due to the motor and power cable properties existing data transmission has traditionally been restricted to DC and low frequency signalling, usually at 1-6 Hz and exceptionally at a few kHz (up to 5).